tree

A directed acyclic graph; i.e. a graph
wherein there is only one route between any pair of nodes,
and there is a notion of "toward top of the tree" (i.e. the
root node), and its opposite direction, toward the leaves.
A tree with n nodes has n-1 edges.

Although maybe not part of the widest definition of a tree, a
common constraint is that no node can have more than one
parent. Moreover, for some applications, it is necessary to
consider a node's daughter nodes to be an ordered list,
instead of merely a set.